Interpretive Summary: The pea aphid causes hundreds of millions of dollars of crop damage every year, and many populations have already acquired resistance towards several commercial insecticides. Although neuropeptides (short chains of amino acids) serve as potent messengers in insects to regulate vital functions, they hold little promise as pest control agents because of susceptibility to being degraded in the target pest and inability to penetrate the outside surface of insects. We report here on the development of versions of insect neuropeptides with enhanced metabolic stability that demonstrates potent aphicidal activity when fed to the pea aphid. Some of the synthetic versions match or exceed the potency of commercial aphicides. The work brings us several steps closer to the development of practical neuropeptide-like substances that will be effective in controlling pest aphids in an environmentally friendly fashion.

Technical Abstract:
The tachykinin-related peptides (TRPs) are multifunctional neuropeptides found in a variety of arthropod species, including the pea aphid Acyrthosiphon pisum (Hemiptera: Aphidae). Two novel biostable TRP analogs containing multiple, sterically-hindered Aib residues were synthesized and found to exhibit significantly enhanced resistance to hydrolysis by angiotensin converting enzyme and neprilysin, tissue-bound enzymes that degrade and inactivate natural TRP peptides. The two biostable analogs were also found to retain significant myostimulatory activity in an isolated cockroach hindgut preparation, the bioassay used to isolate and identify the first members of the TRP family. Indeed one of the analogs (Leuma-TRP-Aib-1) matched the potency and efficacy of the natural, parent TRP peptide in this myotropic bioassay. The two biostable TRP analogs were further fed in solutions of artificial diet to the pea aphid over a period of three days and evaluated for antifeedant and aphicidal activity and compared with the effect of treatment with three natural, unmodified TRPs. The two biostable multi-Aib TRP analogs were observed to elicit aphicidal effects within the first 24 hrs. In contrast natural, unmodified TRPs, including two that are native to the pea aphid, demonstrated little or no activity. The most active analog, double-Aib analog Leuma-TRP-Aib-1 (pEA[Aib]SGFL[Aib]VR-NH2), featured aphicidal activity calculated at an LC50 of 0.0083 nmol/ul (0.0087 ug/ul) and an LT50 of 1.4 days, matching or exceeding the potency of commercially available aphicides. The mechanism of this activity has yet to be established. The aphicidal activity of the biostable TRP analogs may result from disruption of digestive processes by interfering with gut motility patterns and/or with fluid cycling in the gut; processes shown to be regulated by the TRPs in other insects. These active TRP analogs and/or second generation analogs offer potential as environmentally friendly pest aphid control agents.